Vacuum furnaces are used to heat material under conditions requiring an inert atmosphere or a vacuum, frequently to avoid oxidation or other deleterious effects. The heat-treating and brazing of metals and the removal from material of gases absorbed or adsorbed thereinto are examples of procedures in which a vacuum furnace is often required.
Generally, an enclosure is evacuated by means of a diffusion pump or other system for reducing pressure, and provision is made to heat the articles contained in the enclosure. Most commonly, such vacuum furnaces are heated with electric resistance heaters so arranged that heat is radiated to the workpiece. Alternatively, the workpiece may be heated directly or indirectly with an RF induction coil. Direct heating is possible where the workpiece is of a material in which eddy currents can be set up, and indirect heating is achieved by interposing an RF interceptor which is heated by the induced eddy currents and which then radiates heat to the workpiece.
There are certain disadvantages attendant upon the use of conventional electric heating of the types described in vacuum furnaces. For one thing, the cost of electricity is relatively high as compared to the cost of other sources of heat energy such as gas or fossil fuel. Furthermore, it is difficult to obtain favorable heating and temperature distribution in or on the articles within the furnaces using known electrical heating systems. The principal objects of the present invention are to increase the efficiency of vacuum furnaces and to lower the cost of their operation.